Police are investigating the death of an Angolan man who became unwell when security guards attempted to deport him on a flight from Heathrow.

The 46-year-old man was under escort by three civilian guards on BA Flight 77 preparing for departure from Heathrow airport to Luanda in Angola.

Ambulance and police were called to the flight just before 8.30pm on Tuesday. The man was taken to Hillingdon hospital, west London, where he was pronounced dead shortly after arrival. The guards are understood to have been working for G4S, a private firm contracted to assist with deportations for the Home Office.

A police spokesman said: "We know the identity of the deceased but await formal identification and confirmation that next of kin have been informed. A postmortem examination will be arranged in due course. Officers from Heathrow CID are investigating the death.

"At this early stage, we believe the deceased was being deported from the UK under escort by three civilian security guards. Inquiries continue to establish the full circumstances of the incident. There have been no arrests."

There was no reliable information about what led to the man's death or how he became unwell. But in the past, the Home Office's deportation policy has proved highly controversial.

Foreigners can be restrained in deportation flights, although there was no evidence available tonight about the treatment of the man who boarded the British Airways flight.

A UK Border Agency spokesman said: "We can confirm that a detainee was taken ill on a flight preparing to depart from the UK. Paramedics were called and he was taken to hospital where he died.

"The matter is being investigated by the police and the prisons and probation ombudsman."

BA Flight 77 to Luanda, an ordinary passenger flight, was postponed after the man became ill. It left Heathrow tonight at 7pm.

A statement released by G4S tonight suggested the man may have become unwell after the flight had taken off.

"We can confirm that a detainee became unwell whilst being escorted on a flight on 12 October 2010," it stated. "Our staff alerted cabin crew and the plane was immediately returned to Heathrow and the detainee taken to Hillingdon hospital by ambulance.

"Sadly, the detainee passed away upon arrival at the hospital. The matter is now in the hands of the police and we are assisting them with their investigations."

Kevin Wallis, a plane passenger who was sitting near Jimmy Mubenga, says the Angolan deportee was being held with excessive force by security guards when he collapsed and died. Wallis spoke with Paul Lewis by phone from Africa